This invention relates to a method of manufacturing a printed wiring board used in an electronic apparatus.
Referring to FIGS. 1A through 1J, description will be made of a method of manufacturing a printed wiring board as a related art.
Referring to FIG. 1A, preparation is made of a printed wiring board 1 having a plurality of interlayer connection holes. The printed wiring board 1 comprises an insulator substrate 1a, a copper plating layer 1b, and a copper foil 1c. The insulator substrate 1a has through holes formed therein as desired. Then, the insulator substrate 1a is subjected to copper plating to form the copper plating layer 1b on both surfaces of the insulator substrate 1a and inside surfaces of the through holes. The copper plating layer 1b formed on the inside surface of each through hole in the insulator substrate 1a has an inner surface defining the interlayer connection hole of the printed wiring board 1. The interlayer connection holes include a large-diameter through hole 2 having a diameter not smaller than the thickness of the printed wiring board 1 and a small-diameter through hole 3 having a diameter smaller than the thickness of the printed wiring board 1. The printed wiring board 1 prepared as mentioned above is an interlayer connection plating printed wiring board. A hole filling process is carried out for each single interlayer connection plating printed wiring board.
Referring to FIG. 1B, the printed wiring board 1 is placed on a printing table 40 in order to carry out the hole filling process using a printing technique. A printing plate or pattern 50 is used to print a hole filling resin (for example, epoxy resin) 9. A squeegee 70 serves to apply the hole filling resin 9. After the printed wiring board 1 is placed on the printing table 40, the printing pattern 50 is put on the printed wiring board 1 and the hole filling resin 9 is applied and spread on the printing pattern 50 covering the printed wiring board 1 by the use of the squeegee 70.
Referring to FIG. 1C, the hole filling resin 9 is applied and spread on the printing pattern 50 by the squeegee 70. Thus, the hole filling resin 9 is filled in the through holes 3 and 2 as filled resins 19 and 20, respectively. After the through holes 3 and 2 are filled with the hole filling resin 9, the printing pattern 50 is removed and the printed wiring board 1 is displaced from the printing table 40.
Referring to FIG. 1D, the printing pattern 50 is removed from the printed wiring board 1 and the printed wiring board 1 is displaced from the printing table 40. In this state, the hole filling resin 9 is cured. In FIG. 1D, a resin residue 100 is formed by a part of the hole filling resin 9 which has been present in an opening portion of the printing pattern 50 upon filling the hole filling resin 9 in the through holes 3 and 2 and is left on a surface of the printed wiring board 1 or which leaks from the periphery of the opening portion of the printing pattern 50. The resin residue 100 also includes a part of the hole filling resin 9 leaking from a gap between the printing pattern 50 and the printed wiring board 1 in the vicinity of open ends of the through holes 3 and 2. The hole filling resin 9 is cured by heating or photocuring. At this time, both of the filled resins 19 and 20 filled in the through holes 3 and 2 and the resin residue 100 are cured together. As a result, protrusions are formed on the surface of the printed wiring board 1. Such protrusions must be removed by polishing or the like.
Referring to FIG. 1E, the surface of the printed wiring board 1 is flattened and smoothed by the use of a polisher 110. In order to remove the protrusions formed on the surface of the printed wiring board 1 due to presence of the resin residue 100 when the hole filling resin 9 is cured and to flatten and smooth the surface of the printed wiring board 1, polishing is carried out by the use of the polisher 110. At this time, the surface of the printed wiring board 1 is also shaved. As a result of polishing, the printed wiring board 1 as a whole is elongated. Since the copper plating layer 1b and the resin residue 100 different in hardness are simultaneously polished, it is difficult to improve the flatness of the printed wiring board 1.
Referring to FIG. 1F, the protrusions are removed by polishing to produce flattened surfaces 130 and 120 of the hole filling resin 9 filled in the through holes 3 and 2 as the filled resins 19 and 20, respectively. In this state, the through holes 3 and 2 may be referred to as filled through holes. Next, the printed wiring board 1 with the filled through holes is subjected to plating (21 in FIG. 1G) in the following manner.
Referring to FIG. 1G, the printed wiring board 1 with the filled through holes is obtained through the above-mentioned steps and is subjected to plating 21. By the plating 21, the flattened surfaces 130 and 120 of the filled resins 19 and 20 on both sides of the printed wiring board 1 are provided with plating layers 22. Thus, on each of the opposite surfaces of the printed wiring board 1, the copper foil 1c, the copper plating layer (interlayer connection plating layer) 1b, and the plating layer 22 are formed to provide a thick layer. Next, a dry film (23 in FIG. 1H) is adhered onto each surface of the printed wiring board 1 with the plating layer 22 formed thereon to determine a wiring pattern of the printed wiring board 1.
Referring to FIG. 1H, the dry film 23 having a desired wiring pattern is adhered to the printed wiring board 1 plated in FIG. 1G. Thus, the wiring pattern of the printed wiring board 1 is determined by the dry film 23 having the wiring pattern designed for the printed wiring board 1. Next, etching (24 in FIG. 1I) is performed to remove a conductive layer 25 (comprising the plating layer 22, the copper plating layer 1b, and the copper foil 1c) except an area where the dry film 23 is attached, as illustrated in FIG. 1I.
Referring to FIG. 1I, the printed wiring board 1 with the dry film 23 attached thereto in FIG. 1H is subjected to the etching 24. The conductive layer 25 comprises the plating layer 22, the copper plating layer 1b, and the copper foil 1c. After the etching 24, a part of the conductive layer 25 which is protected by the dry film 23 is left to form the wiring pattern (connection pad). Because the conductive layer 25 is thick, etching accuracy is degraded. Then, the dry film 23 remaining on the conductive layer 25 is removed as illustrated in FIG. 1J.
Referring to FIG. 1J, the dry film 23 is removed from the printed wiring board 1 subjected to the etching 24 in FIG. 1I to provide the printed wiring board 1 with the through holes filled and the wiring pattern (connection pad) determined. In FIG. 1J, the connection pads are depicted by 34.
Next referring to FIGS. 2 and 3, description will be made of a merit achieved by filling the hole filling resin in the through holes formed in the printed wiring board 1 as illustrated in FIGS. 1A through 1J.
Referring to FIG. 2, a component 32 is mounted on the printed wiring board 1 in which the through holes are not filled. Referring to FIG. 3, the component 32 is mounted on the printed wiring board 1 in which the through holes are filled.
In FIG. 2, the hole filling resin 9 is not filled in the through hole 3 as the filled resin 19. Therefore, a connection pad to connect the component 32 can not be formed on the through hole 3. On the other hand, in FIG. 3, the through hole 3 is filled with the hole filling resin 9 as the filled resin 19. Therefore, a connection pad 34 comprising the copper plating layer 22 can be formed on the filled resin 19 filled in the through hole 3 in such a manner that the connection pad 34 is connected to the copper plating layer 1b of the printed wiring board 1. Then, the component 32 can be mounted on the connection pad 34 located on the through hole 3.
As illustrated in FIG. 3, the connection pad 34 is formed on the through hole 3 with the filled resin 19 buried therein and the component 32 is directly connected to the connection pad 34 located on the through hole 3. In this manner, the wiring distance is shortened and the inductance is reduced as compared with FIG. 2. Thus, the impedance of the printed wiring board 1 is reduced.
As illustrated in FIG. 3, the filled resin 19 is buried in the through hole 3 so that the connection pad 34 can be formed on the through hole 3 and the component 32 can be directly connected to the connection pad 34 on the through hole 3. Thus, as compared with FIG. 2, the wiring distance is shortened and the inductance is reduced so that the impedance of the printed wiring board 1 is reduced. In this method, however, the complicated steps described in conjunction with FIGS. 1A through 1F are required in order to fill the through hole 3 with the filling resin 9 as the filled resin 19. This decreases the efficiency in production. As illustrated in FIGS. 1A through 1G, a metal layer on the surface of the printed wiring board 1 comprises the copper foil, the interlayer connection plating layer, and the pad. Therefore, the metal layer is thick so that patterning accuracy is inferior.
Furthermore, in the method described in conjunction with FIGS. 1A through 1J, the hole filling process must be followed by polishing as described in conjunction with FIGS. 1E and 1F. Therefore, the productivity is inferior and the printed wiring board 1 is heavily damaged.
It is an object of this invention to provide a method of manufacturing a printed wiring board, which is capable of forming a connection pad on a through hole without carrying out a hole filling process for the through hole.
It is another object of this invention to provide a printed wiring board in which a connection pad can be formed on a through hole without carrying out a hole filling process for the through hole.
According to this invention, there is provided a method of manufacturing a printed wiring board (81), comprising the steps of:
preparing an insulator substrate (1a) having first and second principal surfaces opposite to each other;
forming a first metal foil (82) on the first principal surface;
temporarily fixing a thermosetting resin film (84) to the second principal surface with the thermosetting resin film brought into contact with the second principal surface;
drilling, with the thermosetting resin film temporarily fixed to the second principal surface, a through hole (86) simultaneously in the first metal foil, the insulator substrate, and the thermosetting resin film so that the through hole extends in a direction substantially perpendicular to the first and the second principal surfaces;
simultaneously heating and vacuum-pressing, with a second metal foil (87) brought into contact with the thermosetting resin film after the drilling step, the first metal foil, the insulator substrate, the thermosetting resin film, and the second metal foil to obtain an intermediate printed wiring board in which a bottom (92) of the through hole is covered with the second metal foil and in which the bottom of the through hole has a corner provided with a corner rounded portion (93) formed by the thermosetting resin film so as to protrude from the corner; and
forming a metal plating layer (95) on the first and the second metal foils of both sides of the intermediate printed wiring board, on an inner wall of the through hole, on the corner rounded portion, and on an exposed surface of the second metal foil exposed through the bottom of the through hole to obtain a final printed wiring board provided with interlayer connection.
According to this invention, there is also provided an interlayer connection printed wiring board (81) obtained by the steps of:
preparing an insulator substrate (1a) having first and second principal surfaces opposite to each other;
forming a first metal foil (82) on the first principal surface;
temporarily fixing a thermosetting resin film (84) to the second principal surface with the thermosetting resin film brought into contact with the second principal surface;
drilling, with the thermosetting resin film temporarily fixed to the second principal surface, a through hole (86) simultaneously in the first metal foil, the insulator substrate, and the thermosetting resin film so that the through hole extends in a direction substantially perpendicular to the first and the second principal surfaces;
simultaneously heating and vacuum-pressing, with a second metal foil (87) brought into contact with the thermosetting resin film after the drilling step, the first metal foil, the insulator substrate, the thermosetting resin film, and the second metal foil to obtain an intermediate printed wiring board in which a bottom (92) of the through hole is covered with the second metal foil and in which the bottom of the through hole has a corner provided with a corner rounded portion formed by the thermosetting resin film so as to protrude from the corner; and
forming a metal plating layer (95) on the first and the second metal foils of both sides of the intermediate printed wiring board, on an inner wall of the through hole, on the corner rounded portion, and on an exposed surface of the second metal foil exposed through the bottom of the through hole.
According to this invention, there is also provided a printed wiring board (81) including:
an insulator substrate (1a) having a first principal surface and a second principal surface opposite to the first principal surface; and
a first metal layer (87) formed on the second principal surface;
the insulator substrate having a through hole (86) which is formed therein and extends in a direction substantially perpendicular to the first and the second principal surfaces so that a part of a surface of the first metal layer is exposed as an exposed surface through a bottom (92) of the through hole;
the printed wiring board comprising a second metal layer (95) formed on the first principal surface of the insulator substrate, on an inner wall of the through hole, and on the exposed surface of the first metal layer.